Radiometric imaging uses electromagnetic radiation to obtain images of any kind of objects. By using different frequencies/wave lengths, different images can be obtained and different properties of imaged objects can be obtained. The wave lengths ranging from 1.000 mm to 1 mm are commonly referred to as microwaves. Other definitions mention 300 mm for the upper boundary of the microwave spectrum and 10 mm for the lower boundary. Electromagnetic radiation in the range from 100 mm to 10 mm is commonly referred to as centimeter waves and radiation in the range from 10 mm to 1 mm is commonly referred to as millimeter waves. Submillimeter waves are commonly seen as ranging from 1 mm to 0.1 mm, but may also comprise even smaller wave lengths from the far infrared. Submillimeter waves are also referred to as terahertz radiation. The smaller the wave length, the higher is the attainable resolution. Microwaves, millimeter waves and submillimeter waves penetrate for example clothes or bags and may therefore be used to detect objects hidden underneath.
In the field of radiometric imaging, active and passive receivers (or sensors) exist. Eventually, the object may interact with the emitted radiation by other mechanisms than pure reflection and may, for example, transform, modulate, attenuate etc. the radiation or even change its frequency. The term ‘reflected radiation’ is used to refer to all response radiation emitted, reflected, generated or the like by the object. Active sensors in the microwave spectrum are commonly referred to as radar sensors. However, active sensors could work in the microwave, millimeter wave and terahertz region. Passive sensors (or receivers) sense or receive electromagnetic radiation emitted from an object without generating and transmitting electromagnetic radiation towards the object. An example of a passive sensor is a sensor sensing millimeter waves, submillimeter waves or terahertz waves emitted by an object according to Planck's law (black body radiation) as they are for example used in security screening devices at airports or other menus requiring the checking for contraband objects such as weapons, liquids, bombs, knives, metal, etc. carried under clothes or in bags not being detectable by a human eye.
In order to obtain an image of an object, sensing devices (receiving devices) must scan a two-dimensional field of view. Hereby, it is possible to use one-dimensional antenna arrays having a one-dimensional line of antenna elements or antenna patches which are electronically or mechanically moved in order to create a two-dimensional image. Another possibility to create a two-dimensional image is the use of phased array antennas comprising a plurality of single antenna elements arranged in two dimensions and to perform a full electronic scan by varying the phase of each individual antenna element or patch.
The scanning in order to achieve a two-dimensional image is typically performed by mechanically moving a sharp beam antenna (or a one-dimensional arrangement of sharp beam antenna elements or patches) in the azimuth and elevation directions, where each position corresponds to one pixel of the final image. In order for the sensor or receiver to be effective, it needs to identify the radiated temperature differences between the object to be detected and its surroundings, for example the difference between the human body temperature and the hidden objects underneath the clothes. In order to resolve these temperature differences in prior art systems, the received signal is integrated for each pixel for a fixed period of time, whereby the temperature resolution is inversely proportional to the integration time. In order to achieve sufficient temperature resolution, typical prior art systems are therefore very slow since the sharp antenna beam needs to stay at each position for the duration of the required integration time. Alternatively, some prior art systems achieve a reasonable scanning speed by implementing a high number of parallel receivers which can integrate multiple spots (pixels) in parallel, but which, however, lead to a very costly solution due to the complex structure of the devices.